This invention relates generally to the field of nut and bolt locks and more particularly to a self-locking fastener with a pressure-rolled resilient, deformable plastic patch bonded to the fastener thread surface.
Self-locking threaded fasteners of the type having a locking patch of thermoplastic resin bonded to the thread surfaces have found wide commercial application. This broad demand requires a manufacturing process for high volume production in a rapid and economical manner while maintaining high quality standards. Some important requirements for self-locking fasteners of the type described include sufficient adhesion between the plastic patch and the fastener thread surface, consistent patch material distribution, and controlled patch height. These qualities produce more consistent torque properties than fasteners in which these parameters are less accurately controlled. Inherent in the above requirements is the need to control the dimensional characteristics of the plastic patch in order to produce a uniform patch size and shape for repeatable fastener torque performance. Previous attempts at forming a free-form liquid pool of melted thermoplastic resin on the thread surface of a fastener, such as disclosed in U.S. Pat. No. 3,294,139 to Preziosi and 3,498,352 to Duffy, have certain inherent disadvantages. Included among these disadvantages are difficulty in confining the plastic patch to a definite and predictable configuration, localized buildup of excess patch material which can be easily sheared off, and difficulty in producing a sufficient and predictable amount of adhesion between the plastic patch and the thread surface. Shearing of the material occurs when the fastener containing the plastic patch is engaged in a mating thread causing loosely bonded particles to separate from the threaded surface and finally settle in the workpiece. Examination of fasteners which have been produced by the processes described in the above-noted patents under actual commercial conditions reveal wide variations in the size and shape of the plastic patches, many having an uneven, lumpy surface, or even a "spattered" appearance, with portions of the resin being in the form of small droplets separate from the main plastic body. Any such disparities in physical properties result in corresponding performance disparities, such as unpredictable locking torque values, decreased reusability and possible contamination of the environment in which the locking fastener is used due to the shearing off of excess patch material. Attempts have been made to confine the size and shape of the plastic locking patch, such as disclosed in U.S. Pat. Nos. 3,634,577 to Kull and 3,787,222 to Duffy, but the methods disclosed therein and the resulting locking fasteners produced thereby are not completely satisfactory. Either the process is relatively expensive, such as in the case of the Kull process which requires the use of matching molds or dies brought into contact with the threaded fastener to constrain the free flow of fused plastic material, or the resulting locking fastener does not produce a repeatable, predictable and sufficient amount of locking torque for all applications. For example, Kull's plastic patch material is confined to a height below the thread crests and thus forms an interrupted patch, which is effectively a series of individual patches between thread flanks. Duffy's patch material, on the other hand, extends an uncontrolled distance above the thread crests at the central line of the patch.